Carbon dimer , fluorescence

The effects of the mixed supersonic expansion of CDMA with various solvent molecules (such as cyclohexane, carbon tetrachloride, acetone, acetonitrile, methanol, dichloromethane and chloroform) on the emission spectra have been investigated by Phillips and co-workers [82d[. The cluster size distribution was varied by changing the nozzle temperature and the partial pressure of the solvent. Two emission components were observed in each case. The long-wave emission was attributed to dimers (which can be isolated or solvated) and to monomer complexed with chloroform or dichloromethane (of unknown stoichiometry). On the other hand, it has been reported by Bernstein and co-workers [84] that CDMA forms with acetonitrile two kinds of 1 1 complexes of different geometry. The first cluster has a structured excitation spectrum, similar to that of the bare molecule, but blue shifted by about 252 cm . The second exhibits a broad excitation spectrum with some resolvable features between 31400 and 31 600 cm (Table 2). The complexes show different fluorescence spectra excitation into the broad absorption leads to the red-shifted emission with respect to that of the monomer (Figure 8) and of the blue ... 

Treatment of oxalyl chloride with concentrated hydrogen peroxide under base catalysis in the presence of fluorescers leads to bright chemiluminescence.28 This rather unusual observation was also demonstrated for other oxalyl derivatives (9)29 and interpreted3 to involve 1,2-dioxetandione (3) as the intermediate [Eq. (7)]. Although isolation of the carbon dioxide dimer (3) was claimed,4 no characteristic spectral data could be observed.16 Furthermore, the claim30 of detecting... 

Equation (5) predicts that the appearance of the fluorescence from state 2 should not be instantaneous. When methylene chloride is used as a solvent the fluorescence does have a finite rise time. To show that the state 2 indeed has a finite rise time slower than the apparatus resolution rhodamine b fluorescence is compared to that from the pyrochlorophyll a dimer in carbon tetrachloride in Fig. 30. A finite rise time would not occur from different ground-state molecules in equilibrium. Thus this is additional evidence against two distinct ground-state molecules giving rise to observed phenomena. 

From Table 13.11, the variation of the lifetimes and fluorescence quantum yields in the series of compounds shows the clear increase of homo-chromophore interactions in the excited states when the distance between the chromophores diminishes. The rate and efficiency of the energy transfer in hetero-dimers does not seem to be metal dependent. The distance dependence of the energy transfer rate has been analyzed using Forster and Dexter theories. Harvey and Guilard have established that in 135-Zn-H2 and 136-Zn-H2, energy transfer is dominated by a Forster mechanism, while in the case of hetero-dimers 137, 138, and 139, it proceeds mainly via a Dexter mechanism. The critical distance at which the Dexter mechanism becomes inoperative is estimated between 5 and 6 By analogy with what has been discussed earlier in the case of linearly arranged covalent dimers, it should be noted that for compounds 135-139, no electron density should be present on the meso carbons involved in the covalent connection to the spacer. 

Figure 10. Structures and atom labeling of [2,2]paracy-clophane (Pc), stilbenoid monomers (Ic, 2c) and dimers (lb, 2b). Geometries were obtained from crystal structure data calculated (solid lines) and experimental (dashed lines) absorption spectra and experimental (dotted lines) fluorescence spectra of molecular dimers are shown in arbitrary units. Empirical line width F,. = 0.2ePhas been used to compute absorption profiles contour plots of electronic modes which dominate the absorption spectra of lb and 2b. The axis labels represent the individual carbon atoms as labeled in the molecular templates. Reprinted with permission from ref 92. Copyright 1998 American Chemical Society.

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